The present invention relates generally to temperature controlling devices of the type used to regulate the temperature of a mold in plastic injection molding apparatus and deals more specifically with a temperature controller and alarm device particularly suited for use with a cartridge heater of the general type used in plastic injection molding apparatus.
It is well known that the temperature of a mold in a plastic injection molding apparatus must be maintained within a very narrow range, usually 1.degree.-2.degree. Fahrenheit, in order to produce a high quality plastic molded part that is durable and possesses the expected desired compressive and tensile strengths and other characteristics including visual appearance. In many cases, the mold is heated with a cartridge type heater that is inserted into a receptacle in the mold.
The cartridge heater typically is of the type manufactured by the TEMCO Electric Heater Corporation wherein the heater generally includes a resistive heating element and a thermocouple located within the cartridge housing. Electric power is applied to the heating element to cause it to produce heat proportional to the power applied. The thermocouple is utilized in the normal manner to permit the accurate measurement of a temperature produced by the heater. The amount of electric power supplied to the heater is proportional to the deviation in temperature from a desired temperature and is generally regulated by a closed loop temperature controller. The amount of time that electric power is supplied to the heater is accomplished using well known control techniques, such as, for example, phase angle firing of a triac or silicon controlled rectifier, or controlling the ON-OFF duty cycle of a periodically operated switch to select any desired average power.
Although temperature controllers currently used with plastic injection molding apparatus provide satisfactory regulation to maintain an acceptable temperature deviation from a desired temperature, all of the known controllers fail to address a number of practical problems.
One major drawback associated with known temperature controllers is that the molding process is permitted to continue when an alarm condition indicative of a trouble in the injection molding apparatus occurs. The continued operation often results in the destruction of the mold or molds affected by the trouble. The replacement cost of such a mold is very expensive, often ranging $30,000.00-$35,000.00.
Other problems may cause the production of large quantities of defective molded parts or may cause extensive damage to the temperature controller itself. In some instances, the molded plastic part appears visually to be acceptable; however, if the part is molded at a temperature other than the desired temperature the part may not possess the expected strength and may fail when used.
In addition, increased costs associated with lost production time as well as increased maintenance and repair to the molding apparatus are incurred as a result of the above-identified problems.
Many of the above-mentioned problems result from the various failure modes of a typical cartridge heater used with the plastic injection molding apparatus. For example, in one failure mode, the heating element may short circuit to the cartridge case or to the internal thermocouple associated with the cartridge heater. In another failure mode, the heater may fail to operate due to an open circuit condition caused by a physical break in the resistive element of the heater.
Yet another failure mode is associated with a high impedance or open circuit condition of the thermocouple. Conventionally, a very small current in the range of microamperes is made to flow in the thermocouple and the polarity of the voltage developed across the thermocouple is sensed such that an open circuit condition is indicated as a high temperature and causes the temperature controller to shut down. Since the current must be very small to minimize IR (voltage) drops in the leads connecting the thermocouple, present methods are often only useful in detecting impedances ranging generally from 10 megohms to an open circuit. Consequently, corroded or carbonized electrical connections to the thermocouple may go undetected resulting in inaccurate temperature measurements if the impedance of the thermocouple and its associated electrical circuit is less than the minimum detectable impedance and therefore, the plastic parts may be molded at a temperature other than the desired temperature resulting in the production of defective parts.
Still another problem associated with the failure mode of a heater is the way in which the power is supplied to the heater. Generally, facilities in which temperature controllers are used with plastic injection molding apparatus are serviced by a three-phase electrical power distribution system commonly known as a DELTA configured power system. In such a DELTA configured system, each phase of the three voltage phases is developed across any two of the three conductors forming the DELTA power system. In some instances, a neutral or ground potential conductor is present with the three conductors forming the DELTA power system. The neutral conductor usually carries only unbalanced currents resulting from an electric load, such as, for example, a lighting circuit powered from a WYE configured power system. As is well known, the current carrying capacity of a neutral conductor such as present in a WYE power system is generally much less than the current carrying capacity of any of the conductors comprising the DELTA power system. Additionally, in some facilities serviced by a DELTA power system, the neutral conductor is not present. Consequently, the preferred connection of a heater is line-to-line rather than line-to-neutral to avoid heavy currents in the neutral line.
One problem associated with a heater connected from line-to-line rather than line-to-neutral is that an AC potential is present from each of the line conductors to an electrically grounded neutral. Accordingly, components comprising the heater are also at a high potential with respect to a neutral and additionally, with respect to an electrically grounded frame of the injection molding press, mold and the heater case.
To applicant's knowledge, temperature controllers currently in use connect and disconnect only one voltage line of the two voltage lines across which the heater is connected to remove power from the heater. A major problem occurs, however, when a short circuit develops between the heating element and the heater case. In this failure mode, current flows through the heating element from both voltage lines into the point at which the short circuit has occurred. The additional current flowing through the heating element causes the heater temperature to increase beyond its temperature set point until the temperture controller attempts to remove power to the heater by disconnecting one line from the heater. However, current continues to flow from the other of the voltage lines through the heating element and into the short circuit point. The heater temperature continues to increase due to the current flowing through the heating element to the short circuit point until such time as the controller itself is damaged or the current rises above the rated value of a fuse in series with the voltage line supplyrng current to the heater or until the heating element itself open circuits between the input and the short circuit point.
A number of currently available controllers detect some of the above mentioned fault conditions, such as, for example, an open circuit condition of the thermocouple or a short circuit condition of the heating element. However, these controllers are generally unsatisfactory, do not address all the problems identified above, do not provide adequate alarms, and most importantly do not immediately remove power from the heater.
Often, the only output alarm signal indicating that a fault condition has been detected is in the form of an illuminated panel lamp. A panel lamp warning is generally ineffective since a number of different temperature controllers may be used in any one process, and the illumination of a single panel lamp may go unnoticed or ignored by a busy machine operator. As a result, a fault condition that may drastically affect the quality of a molded part and/or the molding apparatus itself is allowed to persist and worsen.
It is an object of the present invention therefore to provide a temperature controller and alarm device for use with a plastic injection molding apparatus that overcomes the above-described problems.
It is another object of the present invention to provide a temperature controller and alarm device that automatically shuts down operation of a heater when a trouble condition associated with the heating element and thermocouple of the associated cartridge heater is detected.
It is a further object of the present invention to provide a temperature controller for connecting and disconnecting power from both sides of a heating element in a cartridge heater used with a plastic injection molding apparatus.
It is yet a further object of the present invention to detect open circuit and short circuit fault conditions associated with the heating element in a cartridge heater.
It is a yet further object of the present invention to detect and provide an alarm to indicate a high impedance or open circuit fault condition with a thermocouple associated with the heater.
It is a still further object of the present invention to provide a temperature controller having an alarm device that automatically regulates the temperature of a heater to within a small deviation range of a desired temperature.
Additional objects, features and advantages of the present invention will be readily apparent from the following description and drawings forming a part thereof.